There are many reasons why the lymphatic system mayfail, wtih two main groups: primaty and secondary.
Primary (lack of lymphatic vessels): Usually there are simply too few lymphatics; sometimes there are many, but they are very dilated and do not pump properly. This lymphedema can appear before birth, during puberty (which is most common for primaty lymphedema), orlater in life (e.g 30-40).
Secondary (blocked or cut lymphatic vessels): This can be caused by the removal of the lymph nodes as part of the surgery for cancer, often radiotherapy is also used which tends to block the lymph nodes with fibrous tissue. Accidental injuries can tear a major lymph trunk; any excess fibrous tissue caused by the damage can later shrink and constrict them.
In much of the world, the most common cause are tiny filarial worms, injected by mosquito bites, which grow very large and block the lymphatics. The swelling may occur when the worms grow, if they damage the lymphatics and destroy their valves. It usually occurs only some years after the worms are injected - when they die and breakdown. The toxins they then release cause inflammation around the lymphatics, which blocks them.
Lymphedema may also occur because of other diseases. For example, chronic venous diseases almost always involve the large lymphatics, which run beside the large veins; thus lymphedema is superimposed on venous edema and makes it much worse.
Lymphedema is usually considered only as a condition of the arm or leg, but any organ or region may suffer from it. (e.g. the gut, the lungs, the liver as a consequence of chronic hepatitis. etc.).
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